


Deductive Reasoning

by susiephalange



Category: Harry Potter - J. K. Rowling, Sherlock (TV)
Genre: Angst, Deductions, F/M, Fluff, Gryffindor, Hogwarts Seventh Year, Magic-Users, but i use female pronouns, gender neutral reader
Language: English
Status: Completed
Published: 2017-08-17
Updated: 2017-08-17
Packaged: 2018-12-16 15:08:13
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,161
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11831286
Author URL: https://archiveofourown.org/users/susiephalange/pseuds/susiephalange
Summary: Plagued by the ability of abnormally rational thought, Reader has had the ability to deduce since she was young. You could say that it runs in her family. But unlike everyone else who finds her to be an utter asshole, she has her band of friends, the Marauders + Lily, by her side.





	Deductive Reasoning

**Author's Note:**

> I had a prompt from Wattpad for basically a Sherlock!Reader X Sirius Black and I thought I could try and write it a little my own way. Perhaps because I'm not a fan of crossovers, especially if they couldn't logically happen in canon if one was to slip and fall and write it that way. But I wrote this. I hope you guys like it, and stuff. Hope I did my requester justice.

The main reason people did not like you was that you were an asshole. There was no denying the truth; you weren’t very nice sometimes. Maybe it was because you had a way of seeing people and places and things for exactly what they were. The other reason people didn’t like you was that you were Muggle-Born, and back in primary school you’d made Marjory Mallory’s hair turn blue after she told you that you were a freak. It was fair payment, though, because you’d just told her that her mother was cheating on her father with the headmaster’s assistant.

It was these reasons that you had little to no friends. By little, you really meant the only other pests to the order and grand scheme of Hogwarts School of Witchcraft and Wizardry – four Marauders, and a red-headed Gryffindor Head Girl. James, Peter, Remus, Sirius, and Lily. While you were an asshole because of your tenacity and ability of rational reasoning, they were alienated by their will to be different; there was James and Sirius, brothers from separate mothers, Peter, plodding through life to get through another day, and Remus, suffering once a month with the disease he was forced to live out. Lily was a perfectly normal person, but like you, were Muggle-Born, and to the eyes of the supremacists, was therefore subpar.

But friendless or no, you had your fellow Gryffindor associates, and whenever there was a lack in homework (you often finished it before deadline, or even before class ended), you would help the gang of friends with as much mischief as they could manage. Of course, the transfiguration professor, Professor McGonagall was not a fan of your posse losing house points to turn the enchanted ceiling in the great hall into rain clouds (last October) and the incident of the slippery staircase in the Astronomy Tower (last April). Was it worth it? Yes. Mainly because your knowledge and dedication to your studies quickly regained all lost points.

“I was thinking of flooding the Slytherin dungeons…” James grinned, peering over his half-done homework for History of Magic. His glasses were covered in fingerprints, and you wondered how the boy could see out of them even with his terrible eyesight.

Lily coughed into her fist from across the common room, and catching on from his Charms practice, Remus shot a glare at his good friend James. “I don’t suppose it’s necessary, is it?” He asked him. “Unless it’ll give you extra credits. You’re going to fail Professor Binn’s class, and he won’t like that.”

James laughed. “What’ll he do to me, yell ‘ _boo_ ’ _at_ me?”

You glared at James. “No, he’ll force you to repeat the grade again. Your lack of sleep to follow unauthorised extracurricular activities as of late have led to a lack of sleep, which has in turn, led to a lack of motivation and ability to perform in class,” you tell him. “I advise you to sit down, shut up, do your homework, and go to sleep at a reasonable hour, and not taunt the late professor.”

Sirius chuckled from his corner of the common room, but because you were spot on with every part of your observation, there were no more words shared on the subject, and indeed, James sat down, shut up and finished the four feet of parchment required by Professor Binns. Peter sat beside Remus, looking at notes from a Ravenclaw friend for his Herbology essay, but seemed to have trouble with the handwriting. A minute passed within the room in silence, including the students milling in and out from the Fat Lady to their beds upon the late hour. And then, another minute after that.

You let out a sigh, tormented. As much as you wanted to flood the Slytherin common room, perhaps with electric eel infested waters, the wellbeing of your friends plagued your mind. How you were sorted to Gryffindor back in first year, you still knew not, but here you were. Maybe your bravery lay somewhere else. But this noble act of making sure your friends passed their NEWTs was killing you softly.

“I’m going to sleep,” you huff.

Pushing yourself out of your armchair, you go toward the girl’s room. Without waiting to hear your friends’ passing salutations, off you went, and quickly changing into nightclothes, sat cross-legged upon your bed, back bent to allow for your hands to touch both your legs and face as you contemplated, eyes half closed.

“Sickle for your thoughts?”

Opening your eyes, you see Lily walking toward you, rubbing her sore muscles on her shoulders. She has a new smatter of freckles upon her cheekbones, and her feet have an odd step to them. Obviously, her book bag is too heavy, and she’s spending too much time out in the sun watching James play Quidditch, and her shoes are too small.

Instead of telling her of her problems, you take her prompt, and consider your problems. Just last month, you figured out the enigma behind Sirius Black. Ever since, have sworn yourself to not just go and blurt out the very personal things you can see. Not that you have a terrible life at home – you don’t. Your parents are ordinary Muggles, one a mathematician, the other a stay at home father. You had two brothers, and a sister, and as the youngest, saw them all as greater than you could ever hope to be. But nonetheless, you get birthdays, and Christmases, and always get plenty of hugs and kisses when your parents had time around your siblings. You could never picture how hard it would be for Sirius to grow up in a place like that. You were the one who swore him to get out of it, and like the saints they were, the Potter’s took him in.

You shrug.

“Just…busy.” You murmur, flopping backwards on your bed. Your back made a satisfying _crack!_ and smiling to yourself, you ruminate, “Haven’t you figured out which charm to put on your book bag?” Lily hums, confused, and lays beside you on your bed. “Your back.”

“Ah,” she widens her eyes. “How?”

You close your eyes. “I don’t want to talk about it.” You make a noise, that perhaps sounds like halfway between a dying car engine, and a choked cry, “You know, it’s so hard living with myself like this. I – I just want to see things like you guys do.”

Lily turns on her side to look at you. Her green eyes are glittering, and study you before asking, “Is this because you feel like a freak or something?”

You nod.

She shrugs. “We’re already freaks,” Lily breathes. “To my sister, especially.”

* * *

The next time you have spare time not to lay about in the study hall cramming for the upcoming exams, or between meals and bedtime, you find yourself by the Great Lake by the side of the castle. It’s not especially a great lake, per se, perhaps because it’s home to a Giant Squid that wasn’t magical in the slightest was why it was great. You weren’t sure. Perhaps a little research and more staring would bring the answer to that question.

No. That wasn’t what you were thinking. You were at the Great Lake, and all you could think about was how to know as much of it as you could, how to see it as a simple thing and not close your eyes and have nothing on your mind like most of the dull students who floated around as if with no brains within their heads.

You lay on a blanket of your cloak, and in the seasonably warm air, reclined in the half-shade to watch the Squid loll around in the water. Peter is showing James a spell he learned to fix broken things ( _Reparo_ , you learned it ages ago), and Remus is talking to Lily about something to do with Wizarding politics, and beside you, Sirius lay as still as a corpse, enjoying the warmth. If it weren’t for the way his fingers twitched occasionally, you might have thought he was dead, but no. He was very much alive.

“_________,” He turns his head to face yours, those eyes of his staring deep into your soul, “You remember that thing we talked about in July?” By thing, he meant _his home life_ , and by July, he meant _a month ago_. But you didn’t correct him. Instead, you hummed. Taking it as positive, he added, “I wanted to talk to you about how you know. Everything.”

You sit up a little, puzzled. “What do you mean, know everything? I’m only a teenager. I can’t know everything.”

“No, you know what I meant,” Sirius barked a laugh, and scratching behind his ear, corrected, “Your _thing_. You see things and see them for what they are.”

You frown. “My brothers call it deduction. I call it a nuisance. If you think I’m bad, you should see them, they don’t know when to stop. They think I’m sent away to posh boarding school all year to correct me from being a screwed-up kid, but I’m not. I’m here, learning magic.” You sigh. “But you weren’t going to enquire at my personal life, were you? You were probably wanting me to never tell anyone of what I learned.”

Instead of nodding, agreeing, and moving on to another subject, Sirius shakes his head. “I was actually going to tell you something that might help for you.” He sat up, and in turn, fished around in his book bag, and for perhaps the first time in a while, pulled out an actual book from it. “This was in the library.”

The book he hands to you is not modest in size, and seems to be of a good age, suggested by the yellowed pages and worn-out binding. However, it’s not as well-used as the often checked-out books in the Hogwarts Library, as its cover is intact, and has little-to-no dog-eared pages or stains on its exterior. In gold embossed lettering, the script read _The Magic of Logic – the words of Empedocles_ , and running a finger over the words, you felt a rush of power surge from the paper.

“You got me a book?” You ask Sirius. “Don’t tell me you saw it and just grabbed it. Last time you did that you brought _The Complete History of Unicorn Breeding_ with you. Made for some awkward conversation with those Hufflepuff girls.”

Sirius laughed. “No, not at random. A ghost recommended it to me, for you. I think it was the Ravenclaw one.” He frowned. “No. Yes. It was her. I just –,” He looked behind you, to the rest of the Marauders gang. Peter was watching James as he showed him how to hold his wand for that new spell of his. Lily was on a tangent of Muggle politics, and Remus watched in earnest interest in the conundrum of non-magical office. He looked back to you, and moved his hand toward yours, your fingers touching slightly. “I – if it wasn’t for you, then last holidays, I would have had to go back home to that mess I call family. You made me strong enough to leave.”

You swallow a lump that had formed in your throat, and after a beat passes, you find accurate words to converse with. “I – all I did was see you.”

He shrugged, his longer hair swaying in the oncoming autumn breeze. “I’m the family disappointment. You’re the exception to the rule.”

You snort at that. “If I know anything about anything, this is either you wanting to tell me that ‘Rules are meant to be broken’, and therefore a precursor to something else, or…” You look at the young Black teenager before you. You notice that his eyes are trained on yours, and in the warm breeze, has a crop of goose-bumps where his skin is exposed, his eyes dilated, breathing hitched. “…you’re into me.”

He lets out a breathy laugh. “Took you long enough, Holmes.”

Your mind reels at that remark, flashing back to all the encounters with him recently. He’d sit beside you at lunch, dinner. In class, when he could, if not beside James. The two of you would often banter aside the group, or really, as they watched. He backed up your remarks, and trusted you to keep his secret.

Took you long enough? You had missed every sign and symptom of the cause.

“You’re into me?” You repeat.

Sirius nods. “And not just for your stunning good looks. I like how you’re quick to the chase with a scathing word, or three. How you’re always fixing up our mistakes after pranks with the House Points, or talking out of detention…” He laughs a little under his breath. “You’re the coolest person, ever. I, er, wanted to know –,”

You lean forward, and quickly, peck him upon the cheek, which slowly blossoms into a rosy blush. “Yeah, I like you too.”

**Author's Note:**

> If you have any requests, find me on Tumblr at @susiephalange, or [@phalangewrites](https://phalangewrites.tumblr.com/request_conditions) ʕ·ᴥ·ʔ✿


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